Support assemblies

ABSTRACT

In a co-ordinate measuring machine a base structure (10) supports guideways and carriages providing for movement of a probe adapted to contact a workpiece, and a separate structure (20) is provided for supporting the workpiece itself. The structure (20) can be supported directly on a foundation (16) by an arrangement of bars (54) capable of restraining all six degrees of freedom of movement of the structure (20), i.e. straight line movement in each of the X, Y and Z directions and rotation about each of the X, Y and Z axes. Each bar (54) is stiffened in the direction of its own axis but enables compliance in all directions normal to this axis to accommodate small movements of the structure (20) without stress or strain. The base structure (10) is mounted on the workpiece structure (20) by a similar arrangement of the bars (54) which thereby restrain all 6° of freedom of movement of the base structure (10). There is thereby avoided relative movement between the base structure (10) and the workpiece structure (20) which would otherwise result in loss of measurement accuracy to an extent significant in high accuracy measuring and checking machines.

TECHNICAL FIELD

This invention relates to an assembly for supporting elements on afixture and is particularly but not exclusively concerned with a highaccuracy measuring and check machine and the support therein ofmeasuring components and an associated workpiece so as to obviate ormitigate relative movement therebetween.

BACKGROUND ART

Heretofore workpieces have been positioned on machine base structuressuch that any movement of the base structure is accommodated in movementof the workpiece. Conventionally it has been necessary to construct basestructures using heavyweight material such as granite, i.e. materialwhich remains as stable as possible with variations in temperature and,in order to accommodate as far as possible movement in the fixture onwhich the base structure is located, the latter is conventionallysupported on the fixture with minimum restraints. If lightweight basestructures can be utilised it may be that workpieces need not be locatedon the base structure but directly on the fixture, this possibly leadingto relative movement between the base structure and the workpieceresulting in loss of accuracy.

SUMMARY OF THE INVENTION

According to the present invention there is provided apparatus forsupporting elements on a fixture, the apparatus comprising a firststructure for locating at least one of the elements a second structureindependent of the first structure for locating at least another of theelements, at least six elongate bars so mounting one of the structureson the fixture as on the other of the structures as to provide restraintagainst potential space movements of the one structure, and means somounting the other of the structures on the fixtures as to providerestraint against potential space movements of the other structure, themounting means being adapted to obviate relative movement between thestructures due to movement in the fixture, thereby obviating relativemovement between supported elements.

Preferably each of the elongate bars provides compliance in alldirections normal to the bar axis but is stiff in the direction of thebar axis. Further six of the bars may mount said other structure on thefixture and a further six of the bars mount said one structure on saidother structure.

Further said other structure may be mounted on the fixture by anycombination of the bars and anti-vibration mounts totalling six.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings, in which:

FIG. 1 is a schematic elevation of a conventional prior art co-ordinatemeasuring machine;

FIG. 2 is a diagrammatic illustration of a known base structure supportin such a machine;

FIG. 3 shows one embodiment of a component for a support assemblyaccording to the invention;

FIG. 4 is a diagrammatic illustration of a support assembly utilisingthe components of FIG. 3;

FIG. 5 is another embodiment of a support assembly utilising thecomponents of FIG. 3;

FIG. 6 is a schematic elevation of support assembly according to theinvention;

FIG. 6a is a partially broken out perspective view of the supportassembly of FIG. 6;

FIG. 7 is a view similar to FIG. 6 showing an alternative supportassembly;

FIG. 8 is a view similar to FIGS. 6 and 7 but showing a yet furtherexample of a support assembly; and

FIG. 8a is a partially broken out perspective view of the supportassembly of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

A conventional prior art co-ordinate measuring machine as shown in FIG.1 can comprise a base structure 10, which provides stationary guideways12, usually designated as X axis guideways in a rectangular co-ordinatesystem, and a moving structure 14, normally containing the y and Zguideways of the system, and often being a bridge configuration orcolumn configuration. A carriage (not shown) on the moving structure canmount a probe which is adapted to contact a workpiece. The basestructure 10, on which the workpiece is conventionally supported, canitself be supported on a foundation 16 which spreads the load from basestructure supports 18 over a suitable area of sub-soil. For heavystructures, the foundation 16 is usually a thick concrete slab.

The freedom of a body to move in space can be defined as six degrees offreedom, i.e. straight line movement in each of the X, Y and Zdirections, and rotation about each of the X, Y and Z axes. Tocompletely restrain a body against movement it is necessary to restrainall six degrees of freedom by providing a minimum of six suitablypositioned restraints. However it is generally preferred to use theminimum number of restraints, principally in order to accommodate smallmovements of a body relative to its supporting locations, for exampledue to differential thermal expansion, without stress or strain, and toaccommodate small changes in the relative positions of the supportlocations without significant changes in the forces, stresses, strainsand deflections. In the case of the coordinate measuring machine, ifminimum restraints are used, the accuracy of the machine is notsignificantly affected by normal movements of the foundation, whereas ifmore than the minimum restraints are used then foundation movements cannormally seriously affect the machine accuracy. The foundation can movebodily and distort due to changes in the sub-soil, such as change oflevel of the water table, or due to chemical or thermal changes in thefoundation itself.

Conventionally the base structure of a known co-ordinate measuringmachine is supported at three locations by, for example, jacks or aircylinders, whereby if small relative movements occur between the basestructure and the foundation in the X or Y directions, such movementsare accommodated by the compliance of the support locations in thesedirections. Friction at the support locations is often sufficient toprevent large movements.

In other cases the base structure may need to be located accuratelyrelative to some external body or it may need to be restrained becauseof seismic activity or other ground borne forces. To completely locateone body relative to another, six suitable positioned restraints arenecessary. If the base structure is very heavy, the supports may takethe form of large diameter carbide or ceramic spheres 19 engagingsurfaces of similar materials. At two of the support locations thespheres may locate respectively in a vee slot and a trihedral hole inthe base structure, with the sphere at the other support locationengaging a plane surface on the base structure (FIG. 2). The spheres canbe rigidly attached to the foundation by bolts or the like, or byseating the spheres in trihedral recesses in the foundation. Thisprovides the six restraints required.

In a co-ordinate measurinq machine, the base structure 10 must be madestiff enough to have a negligible deflection under imposed forces whichinclude the weight of the base structure 10 itself, the weight of themoving structure 14, the weight of the workpiece with associatedcomponents, and the reactions at the support locations, below the basestructure 10, to each of these active forces. If the moving structure 14is light in weight and a less stiff base structure 10 is used toadequately support the moving structure 14, for example where themachine has a structure described in our co-pending Application entitled"Machine Structure", then if the workpiece is heavy, the deflection ofthe base structure 10 due to the workpiece will not be negligible. Tomeet this problem it is possible in principle to support the workpieceon a separate workpiece structure which does not apply any forces to thebase structure 10. The workpiece structure can be independentlysupported by the foundation 16 but it is essential that no significantrelative movement occurs between the workpiece structure and the basestructure during duration of one set of measurements. Any distortion ofthe foundation 16 will cause relative movement and significantdistortion can occur in a few hours. A worse problem can occur if thebase structure 10 must be isolated from ground borne vibrations bysupporting the base structure on compliant supports between thefoundation 16 and the base structure 10.

In a support assembly of the present invention, supports in the form ofbars, i.e. ties or struts, can be used at the support locations.Referring to FIG. 3, there is shown a bar 54, for example of circularcross-section, which has reduced diameter sections 56 towards respectiveends defining hinges which, while the bar 54 is stiff in the directionof its own axis, enables compliance in all directions normal to thisaxis. To avoid crippling or buckling of the bar 54 under a compressiveforce, a tensile force can be applied to the bar by means of springs,air cylinders or other suitable means. Thus the weight of the basestructure 10 could be supported by springs or the like, leaving the bars54 to perform the function of location only, whereby the bars 54 can besufficiently compliant to apply negligible forces to the base structure10 normal to the bar axis. Alternative constructions of the bars 24 areof course possible and reference in this connection is made to ourco-pending Application entitled "Linear Guiding Apparatus".

FIG. 4 shows schematically one arrangement of bars 54 providing for sixrestraints simulating the arrangement of a trihedral recess, vee slotand plane contact surface. FIG. 5 shows schematically an alternativearrangement of bars 54 arranged in three pairs with each pair providingone linear and one angular restraint.

To accommodate the separate workpiece structure 20, components of thebase structure 10, as shown in FIG. 6, may be designed to accommodatethe arrangement of bars 54 referred to in FIG. 5 (e.g. see FIG. 6a). Thebase structure components and the workpiece structure are provided withextension parts between which bars 54 can be located. The verticalweight forces transmitted between the base structure 10 and theworkpiece structure 20 are in direct line with the vertical weightforces transmitted between the workpiece structure 20 and the foundation16.

In an alternative arrangement as shown in FIG. 7 the workpiece structure20 is supported on the foundation 16 through anti-vibration mounts 58.An anti-vibration mount is a piece of compliant material interposedbetween the foundation or fixture and the structure to prevent orattenuate the transmission of vibrations from the fixture to thestructure. It provides vibration isolation and vibration clamping. Theisolation implies elasticity and the vibration clamping implies energyabsorption. Spherical supports 22 locate in respective trihedral and veerecesses and engage with a plane contact surface as

described, while the base structure hereinbefore described, while thebase structure components are supported on the workpiece structure 20 byan arrangement of the bars 54. In a further alternative arrangement asshown in FIG. 8, the workpiece structure 20 is supported on thefoundation 16 via the mounts 58 by a suitable arrangement of six of thebars 54 similar to the arrangement shown in FIG. 4, while the componentsof the base structure 10 are supported on the workpiece structure 20 bya similar arrangement of six of the bars 54, thereby giving the sixdegrees of restraint. FIG. 8a shows the support assembly of FIG. 8 inpartially broken out perspective for clarity. Again however because ofthe use of the vibration isolation mounts 58, the lower six bars 54 canbe replaced by nine bars 54 arranged in three tripods. While theworkpiece structure 20 can be supported on the foundation 16 by means ofthe spheres 22 as shown in FIG. 7, or a six bar arrangement as shown inFIGS. 6 and 8, it is to be appreciated that the structure 20 can besupported on the foundation 16 by other arrangements. For example, theremay be provided a combination of three bars and three anti-vibrationmounts, or six anti-vibration mounts, or any combination of bars andanti-vibration mounts totalling six. When a single anti-vibration mountcan deal with both horizontal and vertical vibrations, then a total of 3anti-vibration mounts may be adequate without any additional mounts orbars. Further the workpiece structure may be Provided with three veerecesses as an alternative to the arrangement described with referenceto FIG. 7. In a further alternative the spheres may locate in conical ortrihedral recesses in the workpiece structure 20 and either onearrangement of a conical or trihedral recess, a vee recess and a flat onthe foundation 16, or three vee recesses on the latter.

It is also to be appreciated that, although the workpiece structure 20has been described and shown as being supported on the foundation 16with the base structure 10 supported on the workpiece structure 20, thelatter may in fact be supported directly on the foundation 16 and inturn support the workpiece structure 20.

It is also possible for each of the base structure 10 and the workpiecestructure 20 to be mounted directly on the foundation 16. In thisarrangement each structure is supported by an arrangement of the bars 54as hereinbefore described.

When the method of using the bars 54 is applied to heavy structure,there can be a problem in providing sufficient strength to support thestructures while allowing sufficient lateral compliance to avoidunacceptably large lateral forces, and also having acceptable bendingstresses in the bars. For this reason there are used bars of adequatecross-section to support the weight but of considerable length to reducethe lateral forces and the bending stresses for any given lateraldisplacement. In the case of struts, it is proposed to use largediameter bars of cylindrical or Prismoidal shape with a reduced sectionat each end to act as an elastic or plastic hinge.

By separating the base structure 10 which carries the accurate measuringdevices, from the workpiece structure 20 carrying the workpiece andassociated components and any other heavy items which may vary with eachworkpiece, and locating the base structure 10 and the workpiecestructure 20 relative to each other by no more than six properlyoriented restraints, relative movement between the base structure 10 andthe workpiece structure 20, which would otherwise result in loss ofmeasuring accuracy, can be avoided.

Various modifications may be made without departing from the invention.For example different arrangements of spherical supports and barsupports can be used and indeed many combinations of the sphericalsupports and the bar supports can be utilised.

As mentioned hereinbefore, the support apparatus of the invention isparticularly suitable for use with a measuring machine having astructure as described in our co-pending U.K. Application No. 8808280entitled "Machine Structure", and can also benefit from a temperaturecontrol arrangement as described in our co-pending U.K. Application No.8808281 entitled "Support Structures".

I claim:
 1. An apparatus for supporting a plurality of elements on afoundation, said elements comprising a workpiece structure forsupporting a workpiece and a base structure, said apparatus comprising afirst means having a plurality of components for supporting either theworkpiece structure or the base structure defining a supported elementon said foundation. and a second means for mounting the other of theworkpiece structure of the base structure defining a mounted element onthe supported element, said first supporting means restraining saidsupported element against both linear and angular movement relative tosaid foundation, and said second mounting means comprising six elongatebars for restraining the mounted element against linear movement alongand rotation about three mutually perpendicular axes relative to saidsupported element, thereby obviating relative movement between theelements and said foundation.
 2. The apparatus of claim 1, wherein eachof the elongate bars has a bar axis along its longitudinal length, andwherein each bar allows limited movement in all directions normal to itsbar axis but is stiff in a direction parallel to such axis.
 3. Theapparatus of claim 1, wherein said first supporting means comprises sixelongate bars for supporting said supported element on said foundation,said elongate bars restraining the supported element against linearmovement along and rotation about three mutually perpendicular axesrelative to said foundation.
 4. The apparatus of claims 1 or 2 whereinsaid first supporting means comprises a plurality of spherical supports.5. The apparatus of claim 4, wherein said supported element comprises avee recess and a trihedral recess each receiving and locating aspherical support, and wherein said foundation comprises recesses forreceiving and locating said spherical supports in spaced relation. 6.The apparatus of claim 5, wherein a third spherical support is receivedand located in a corresponding recess in said foundation and engages asubstantially planar surface of the supported element.
 7. The apparatusof claim 4, wherein said supported element comprises recesses forreceiving and locating two spherical supports, and wherein saidfoundation comprises a v-shaped recess and a separate trihedral recessfor receiving and locating said spherical supports in spaced relation.8. The apparatus of claim 7, further comprising a third sphericalsupport received and located in a corresponding recess in said supportedelement and engaging a substantially planar surface of said foundation.9. The apparatus of claim 4 wherein said spherical supports are receivedand supported in respective vee recesses formed in said supportedelement and either conical or trihedral recesses formed in saidfoundation.
 10. The apparatus of claim 4, wherein said sphericalsupports are received and supported in respective vee recesses formed insaid foundation and either conical or trihedral recesses formed in saidsupported element.
 11. The apparatus of claim 1, wherein said firstsupporting means comprises any combination of elongate bars andanti-vibration mounts totalling six.
 12. An apparatus for supporting aworkpiece structure and a base structure on a foundation, said apparatuscomprising a first means having a plurality of components for supportingeither the workpiece structure or the base structure defining asupported element on said foundation, and a second means for mountingthe other of the workpiece structure or the base structure defining amounted element on the supported element, said first supporting meansrestraining said supported element against both linear and angularmovement relative to said foundation, and said second mounting meanscomprising six elongate bars for restraining the mounted element againstlinear movement along and rotation about three mutually perpendicularaxes relative to said supported element, each of said elongate barshaving a longitudinal bar axis and being relatively compliant in alldirections normal to said axis, but rigid in the direction of said axis,thereby obviating relative movement between the elements and saidfoundation.
 13. The apparatus of claim 12, wherein said bars compriseopposite ends, and a reduced diameter section adjacent each end whichacts as an elastic hinge to allow compliance in directions normal to thebar axis.
 14. The apparatus of claim 12, wherein said first supportingmeans comprises six elongate bars for supporting said supported elementon said foundation, said elongate bars restraining the supported elementagainst linear movement along and rotation about three mutuallyperpendicular axes relative to said foundation.
 15. The apparatus ofclaim 12, wherein said first supporting means comprises a plurality ofspherical supports.
 16. The apparatus of claim 12, wherein said firstsupporting means comprises any combination of elongate bars andanti-vibration mounts totalling six.
 17. An apparatus for supporting aplurality of elements on a foundation, said elements comprising aworkpiece structure base structure, said apparatus comprising a firstmeans for supporting either the workpiece structure or the basestructure defining a supported element on said foundation, and a secondmeans for mounting the other of the workpiece structure or the basestructure defining a mounted element on the supported element, saidfirst supporting means comprising six bars restraining said supportedelement against both linear and angular movement relative to saidfoundation, and said second mounting means comprising six elongate barsfor restraining the mounted element against linear movement along androtation about three mutually perpendicular axes relative to saidsupported element of said foundations.
 18. The apparatus of claim 17,wherein each of the elongate bars has a bar axis along its longitudinallength, and wherein each bar allows limited movement in all directionsnormal to its bar axis but is stiff in a direction parallel to suchaxis.